The cheap radar modules sens through walls and have no directionality.

The easy and cheap sensor methods you can rely on to sense if someone is standing in front of a picture frame is either reflective IR or ultrasonic sensor if the room is of reasonable size (more than 1 meter between facing images). First, an ultrasonic sensor needs an echo within a narrorange of time (20 mSec) to create an effect. You can use hardware or a microcontroller and software to insure an object is less than 1 meter (viewing distance) and anything greater would be ignored. There is little chance of any cross-talk on the cheap ultrasonic sensors if sensors are more than 3 meters apart. The only time you'll have trouble is if the sensor fires in the same 20-millisecond window - AND - the two sensors are within the 3 to 4-meter line-of-site.

IR can be used like the modulated IR on a garage door opener safety system. It becomes quite easy if you can reflect off of a white ( light colored) wall across from the picture frame. Alternatively, you can reflect off of the person standing and looking at the picture frame. This has some challenges because dark clothing and/or dark skin tones tend to absorb near-IR used by these sensors.

You can also make a simple beam interrupt sensor if you have any sort of gate systems to keep people a few feet back from the picture frame. You can put a second set of gates behind the viewer and use a send + receive to sense when someone is standing in front of the sensor and interrupting the beam.

 

The cheap radar modules sens through walls and have no directionality.

Radar sensors like the one I linked can easily be made directional and have a sensitivity adjustment. A shield can be fabricated using ordinary copper clad PCB restricting back and side sensing. The sensitivity adjustment can limit the range to a meter or so.

The ease of triggering the sensor makes it almost like a presence detector. The person would have to be completely still for the timeout period for it to turn off. By default they feature re-triggering so every motion extends the time, which is adjustable via a capacitor.

 

Why not just use a SONAR sensor instead?

I can't use sonar, because I will have several devices in the same room and they interfere with each other.

Put a delay on the signal so that the music stays on for some fixed time after it detects motion.

This task requires of me to cut the music when the person steps away, a delay will not suffice.

I think using a sensor to detect a reflected modulated IR beam would be a better solution now that you have described what you are trying to achieve.

I was thinking of using a TOF sensor, but will they not interfere with one another in the same room? Do you have a specific recommendation?

I think radar sensors like this one are much more suited to presence detection.

Thanks will check it out, but do you think that several in a room might interfere with one another?

hi MT,

The PIR module I often use, inexpensive, has Sensitivity and On Time Period trim pots.

Yes I have tried that sensor, it specifically detects motion, and you must move quite a lot to trigger it.

 

The cheap radar modules sens through walls and have no directionality.

Do you think that a thermopile like this could work? Or a one channel PIR sensor like the PM611 seen here (or with one channel occluded)?

 

Do you think that a thermopile like this could work? Or a one channel PIR sensor like the PM611 seen here (or with one channel occluded)?

They are the same as the first device you mentioned.

 

hi,
You can get pressure mats, thin enough to fit under a carpet.

E
https://www.google.com/search?client=firefox-b-d&q=under+carpet+pressure+mat

 

Thanks will check it out, but do you think that several in a room might interfere with one another?

They might, you’d have to test. But the way they operate I suspect they would be immune.

However, I have a different suggestion. It is old school but given your requirements might fit the bill better.

An electromechanical pressure mat, like these. Others are available probably a lot cheaper. This is a venerable technology and if you can place it properly, a lot less finicky than any other sort of sensor. I have, on occasion, made my own switches using a panel (for example of tempered hardboard activating a switch in the center.

 

hi,
You can get pressure mats, thin enough to fit under a carpet.

E

Ha. I was just finishing my message when the “new message” notification appeared.

 

hi,
You can get pressure mats, thin enough to fit under a carpet.

E
https://www.google.com/search?client=firefox-b-d&q=under+carpet+pressure+mat

This is the solution I used for several effects in haunted houses.

I also used PIR devices with a time delay before triggering the effect and a variable time when then effect was triggered.

In fact, someone hired me for an art piece, to trigger an animation sequence, which ran for a minute or two, whenever motion was detected. A simple, cheep PIR sensor was satisfactory for the art gallery.

 

Thinking about PIR detectors in general, if you look at the output of the raw detector the signal is a differentiated measurement of the temperature of the sensor. When a warm body gets into the field of view, there will be a spike in one direction and when the warm body leaves the field of view there will be a spike in the opposite direction. This means that if you watch long enough you can tell when the field of view is occupied.

- OR -

In a fixed installation an emitter can be bounced off of a reflector or mirror back to a detector. Driving the transmitter with pseudo noise and using the same pseudo noise signal to demodulate the reflected signal can be done to prevent the receiver from detecting signals from other transmitters.

 

I think radar sensors like this one are much more suited to presence detection. They are cheap, very effective, and require much less motion than a PIR sensor. All the commercial occupancy sensors I know of require motion to activate.

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You are right Ya'akov, but they are also very very sensitive and not particularly focused, so if there are others in the room and are in the range of the device, or in some cases in the adjacent room, will trigger the device. Perhaps the sensitivity can be adjusted on some units with a change of a resistor value.

 

VL53L0X Time-of-Flight (ToF) Laser Ranging Sensor Breakout 940nm GY-VL53L0XV2 is a simple easy sensor to use.